Vapor deposition apparatus

ABSTRACT

An apparatus for producing monocrystalline deposits on a substrate by vacuum evaporation. The source of vapor to be deposited is a rotating cylinder which is off center with respect to an electron beam which strikes it and causes vaporization. The chamber walls are grounded and the substrate is held at a low positive potential rather than a high negative potential as is the usual practice.

United States Patent Vanderschueren [4 1 Sept. 26, 1972 [54] VAPORDEPOSITION APPARATUS 3,344,054 9/1967 Laegreid et a1 ..1 18/49.5 Xinventor: Emile vanderschueren Schagen 3,347,701 Yama gishl et a1. l XNetherlands 3,361,591 1/1968 D111 et a1. ..118/49.1 X 3,375,804 4/1968Rausch ..118/49.1 X [731 Asslgnw European Atomic Energy Communi-3,492,969 2/1970 Emeis ..118/49.1 (Emmm- Brussels 3618mm 3,552,3521/1971 McConnell ..118/49.5 [22] Filed: April 22, 1969 3,330,251 7/1967Gutsche ..1l8/49.5 2 3,437,734 4/1969 Roman et a1 ..118/49.5 X [21] APPL8181270 2,932,588 4/1960 Frank ..1 18/491 x 2,771,568 11/1956Steigerwald ..1 18/495 X [30] Foreign Application Priority Data 3Primary Examiner-Morris Kaplan 8 B l ..60325 June 196 e glumAtt0rney-Stevens, Davis, Miller & Mosher [52] US. Cl ..ll8/49.5 511 1m.(:1 ..C23c 15/12 [571 ABSTRACT I 0f vnvlonool An for 117/9331 on asubstrate by vacuum evaporation. The source of vapor to be deposited isa rotating cylinder which is [56] References C'ted off center withrespect to an electron beam which UNITED STATES PATENTS strikes it andcauses vaporization, The chamber walls are grounded and the substrate isheld at a low posi- 2,391595 l2/1945 Rlchards "118/49 X tive potentialrather than a high negative potential as 3,192,892 7/1965 Hanson et a1..118/49.l is the usual practica 3,243,174 3/1966 Sweet ..1 l8/49.5 X3,293,074 12/1966 Nick] ..117/106 A X 1 Claim, 1 Drawing Figure VAPORDEPOSITION APPARATUS The invention concerns apparatus on depositing, byevaporation ina vacuum of at least Torr, monocrystalline layers ofmaterial on substrates of any kind and shape, and a crystallizing vesselor apparatus for carrying out this method. The term monocrystallinelayer is here used in a broad sense: it means both a truemonocrystalline layer, in which the deposit formed consists of a singlemonocrystal, and a layer consisting of monocrystals all having the sameorientation (epitaxy), as opposed to a polycrystalline layer consistingof monocrystals having all possible orientations.

In the present state of the art, there are many methods of producingdeposits of material on various substrates in a vacuum. These includethe various types of spraying, including high-frequency spraying, andvacuum evaporation. Deposits formed by these methods have variableproperties of adhesion, quality and rate of deposition and it is anobject of the invention to provide an apparatus which enables the degreeof adhesion to be controlled, which provides a high deposition rate andwhich enables a monocrystalline deposit crystallized along the densestlattice planes to be obtained.

The invention is characterized in that the material which is to bedeposited is evaporated in a vacuum of at least 10 Torr by heating it toits boiling point, the vapor thus produced is concentrated around thesubstrate and this vapor is ionized and deposited on the substrate towhich is applied an electrical potential capable of attracting theionized vapor but not interfering with ionization, the free surface ofthe substrate, during a first period necessary for formation of anadhering layer, being at a temperature as near the critical temperaturefor adhesion of the material evaporated on to the material of thesubstrate as is desirable for the quality of the adhesion required(which is greatest at the said critical temperature for adhesion) and,for the remainder of the deposition period, at the critical temperaturefor crystallization of the vapor and surfaces on which no deposition isdesired being brought at least to the critical temperature for recoilfor the vapor concemed.

The invention is based on the selection of a group of relativelycritical operating conditions (temperatures and electric filed). First,it should be noted that the vapor of material to be deposited comes froma source where this material has been heated to its boiling point. In avacuum, using this vapor, the Applicants have found that certainphenomena take place at definite temperatures already mentioned andwhich are defined below and at certain electric field values.

a. The critical temperature for crystallization of the vapor coming froma source at is boiling point. The Applicants have found that the atomsor molecules coming from a source in which the material to be depositedis brought to its boiling point are deposited on an existing crystallattice of this material and continue this lattice or cover it with amonocrystalline layer, provided that at least the layers nearest thefree layersof this existing lattice are at a critical temperature whichthe Applicants call the critical temperature for crystallization. TheApplicants have found the value of this critical temperature for varioussubstances. By way of example, the critical temperature forcrystallization, in the operating conditions defined above, is aboutl000C for nickel, 1,350C for alumina, 230C for cadmium and l,500C formolybdenum. As a general rule, and judging from the technical resultswhich have been obtained, this critical temperature for crystallizationis between six-tenths and seven-tenths of the melting point for thematerial.

When the existing crystal lattice (formed either by the originalsubstrate if this is of the same kind as the deposit, or by the vaporalready crystallized by the method according to the invention), or atleast its free layers, are at the critical temperature forcrystallization, the incident atoms or molecules (from a source atboiling point) are deposited on this lattice and rearrange themselves inorder to continue it and extend it.

b. The critical temperature for recoil. The atoms or molecules of thevapor to be deposited, come from a source at its boiling point andreaching any surface, recoil from this surface without being depositedon it when it is at a temperature at least equal to the criticaltemperature for recoil. This critical temperature for recoil is betweenthe critical temperature for substrate, defined above, and the meltingpoint for the material to be evaporated. ln apparatus for carrying outthe invention, all surfaces where no deposit is desired are brought tothis temperature, so that the atoms or molecules of vapor which touchthese surfaces are reflected and remain available for deposition on thesubstrate. In the operating conditions for the invention, the recoiltemperature for cadmium is about 290C and that for alumina about 1,600C.

c. The critical temperature for adhesion of the crystallized layer toits substrate. Where the substrate is of the same kind as the deposit,the deposit will of course adhere best to the substrate if the latter isat the critical temperature for crystallization. The deposit thencontinues or extends the structure of the substrate. In the most generalcase; in which the substrate and deposit are of different kinds, thereis for the substrate, or at least for its free surface, a criticaltemperature for adhesion of the deposit. This temperature is at leastequal to the critical temperature for crystallization of the material tobe deposited and is less than the recoil temperature.

When the substrate is brought to this adhesion temperature, the atoms ofthe incident vapor penetrate into the substrate to a depth of a fewlattice spacings and arrange themselves along their own lattice, whichoverlaps or interpenetrates with that of the substrate or, more simply,with the material of the substrate if the latter is not in crystallineform. Adhesion is very good. At this level, one finds a compound of thesubstances constituting the substrate and the deposit. Going from thesubstrate towards the deposit, this compound starts by being very poorin atoms of the vapor, but becomes progressively richer in these atomsuntil it does not contain any of the substrate material.

Deposits obtained in accordance with the invention are formed in avacuum enclosure (in a vacuum of at least 10" Torr) by heating thematerial to be deposited to its boiling point. The free surface of thesubstrate is brought to its critical temperature for adhesion of thevapor concerned long enough for the adhering layer to form.

if the aim is solely to obtain an adhering layer consisting of acompound of the materials constituting the substrate and the vapor,deposition can stop when there are no longer substrate atoms availableto form the compound. Layers of compound several microns thick have beenformed in this way, for example in order to make a corrosion-resistantlayer.

If, on the other had, it is desirable to obtain a deposit of theevaporated substance when the adhering layer no longer contains anythingbut the atoms from the vapor, disposed along their own lattice, the freelayer of the substrate obtained is brought to the critical temperaturefor crystallization of the vapor. The vapor is then deposited, forming amonocrystalline layer.

Advantageously, in order to increase the rate and quality of depositionand its adhesion, the density of the vapor'around the substrate isincreased. To this end, the vapor is concentrated around the substrateby any known means, and all the surfaces on which no deposit is desiredare brought at least to the temperature for recoil of the vapor atoms ormolecules. These atoms or molecules are then available only fordeposition on the substrate, and material losses are greatly reduced.Very simple and very effective means for concentrating the vapor will bedescribed later in this specification.

A spectacular improvement in both the yield and the rate of depositionand in the adhesion and depth of the deposit is obtained by ionizing thevapor particles and applying to the substrate a potential of oppositesign to that of ionization. This potential maintains ionization of thevapor, directs and accelerates the atoms or molecules of the ionizedvapor towards the substrate. and carries out an ion pumping action. Thelatter consists in that the atoms or molecules foreign to the materialto be deposited are ejected from the vapor. This potential is effectivefrom low voltages (of the order of some tens of Volts in experimentalconditions), and it has an upper limit due to its influence on thestability of the means for heating and ionizing the vapor to bedeposited. In practice,-during normal operation, it is between 10 and120 Volts, but it may be higher.

The method is preferably carried out by ionizing the vapor atoms ormolecules with an electron beam (coming from a gun or from an emissionsource) and applying to the substrate a potential higher than that ofthe surrounding space. The rate of deposition can now be of the order of1 mm per hour and also this is important and does not happen withoutthis field the deposit obtained is crystallized along the densestlattice planes. The density and quality of the deposit are thereforemuch increased. As regards the adhering layer, the inter-penetrationwith the material of the substrate is improved, as is its depth, so thatthe qualities of adhesion of the deposit itself are, of course, alsoimproved.

If, on the other hand, a detachable deposit is desired, it is merelynecessary, during a first stage, to avoid bringing the substrate to thecritical temperature for adhesion and applying an electrical potentialto it. As soon as a monatomic layer has covered the substrate, thelatter is subjected to the electric-field and temperature conditionsdescribed above with reference to obtaining high-quality monocrystallinedeposits. The deposit is is then easily separated from the substrate.The speed at which this deposit forms means that the invention can beused for making sheets or foil of very high quality (beingmonocrystalline), which can be detached from the substrate by hand.

Some mechanical elements of complex shape can also be made in the formof a monocrystalline layer deposited on a substrate of suitable shapewithout adhering to this substrate. Elements made in this way are formedof material in a much more perfect state than those obtained byconventional metallurgical means, which cause dislocations in thematerial.

The invention can also be used very advantageously for makingmonocrystals of .any size or shape.

The method just described also makes it possible to vary the thicknessof the deposits locally by placing grounded screens opposite placeswhere no or a decreased deposit is desired. For deposits of complexshape, it is even possible to provide grounded screens containingapertures with the shapedesired for the deposit, in the manner of thestencils used in painting.

The present invention can also be used for forming deposits from vaporfrom a plurality of sources and from different vapors from differentsources.

The method just described also makes it possible to form welds ofexcellent quality between elements which cannot be welded by knownmethods. In accordance with the invention, the welding material isapplied in the form of a vapor from a source in which it has beenbrought to boiling point. Each of the elements to be welded is, at thelevel of the connection which is to be made, brought to the criticaltemperature for adhesion of the vapor for the material constituting it.A grounded screen containing an aperture opposite the place where theweld is required may advantageously be used as described above. As soonas the adhering layers are completed, their free surfaces are brought tothe critical temperature for crystallization of the vapor, and amonocrystalline deposit of welding material therefore forms whichconnects the two elements and adheres strongly to both of them.Metal/ceramic welds of very high quality have been made in this way,although the elements to be welded have been left a at fairly lowtemperatures.

The apparatus according to the invention is characterized in that itcomprises a closed chamber situated inside. the vacuum enclosure andcontaining at least one source of vapor of the material to be deposited,where the latter is brought to its boiling point, and a substratesupport, heating means for bringing the walls of the chamber to'thetemperature for recoil of the atoms of molecules of the vapor from thesource, and substrate heating means capable of bringing the free surfaceof the substrate either to the temperature for adhesion of the vaporfrom the source or to the temperature for crystallization of this vapor,as required.

A crystallizing vessel according to the invention and for carrying outthe method described above and its use will now be described withreference to the accompanying drawing which represents a diagrammaticview of the vessel.

The device shown in this FIGURE is inside a vacuum enclosure (not shown)in which a vacuum of at least 10" Torr can be produced.

The Figure shows a source of vapor of the material to be deposited, inthe form of a cylindrical block 1 having a vertical axis and composed ofthe material to be deposited. This block 1 is mounted at the upper endof a vertical shaft 2 coaxial with it. The source is grounded by meansof a connection 3. The vertical shaft 2 is rotated by a motor (notshown) at a speed of 2 to 5 revolutions per minute.

The source, or the shaft supporting it, extends into a chamber 4 whichis grounded by means of a connection 5. An orifice 6 in the wall of thischamber permits passage of an electron beam from a gun (not shown)situated in the vacuum enclosure outside the chamber 4. The wall of thechamber also has an orifice 7, through which a rod 8 supporting asubstrate 9 passes. The rod 8 is fixed or freely movable in respect ofrotation and/or translation, and it is electrically connected by a lead10 to the positive pole of a direct-current generator 11, whose negativepole is grounded. By means of a variable resistor or potentiometer 12,the potential difference between the substrate and the surrounding spacecan be adjusted as desired between zero and a voltage which does notseriously disturb the electron beam, usually between 10 and 120 Volts.The electron beam passing through the orifice 6 in the chamber wallbombards the flat upper surface of the cylinder 1. It is so directedthat its point of impact is at a distance of the order of a fewmillimeters from the center of this face. Because the shaft 2 isrotating the cylinder 1, the point of impact of the electron beam shiftscontinuously. Its locus is a circle 13. As a result, the electron beamdoes not form a'deep, narrow hole in the source: the heat is betterdistributed and melts, on the cylinder 1, a small mass of material to beevaporated, which mass is situated at the center of the upper surface ofthe cylinder and is shaped like a lens defined approximately by thecircumference 13. There is therefore a fairly large and flat surface forevaporation of the liquid, permitting rapid and homogeneous emission ofvapor.

The walls of the chamber 4 are brought by any known means to thetemperature for recoil of the atoms of vapor from the source or to atemperature as near the recoil temperature as is desired. Other means(also not shown) can bring the free surface of the substrate to thecritical temperature for adhesion or to the critical temperature forcrystallization of the vapor, or even if the adhesion of the deposit isto be very poor to a temperature remote from these temperatures.

Because of the chamber 4, and provided that the apertures in thischamber provide only a small crosssection for the vapor from the sourceto escape, this vapor cannot disperse into the vacuum enclosure, butrecoils from the chamber walls and remains concentrated around thesubstrate 9.

Also, the peripheral electrons of the beam heating the source ionize thevapor atoms or molecules, so that when the substrate is polarizedrelative to the space surrounding it these atoms or molecules areconcentrated and attracted to the substrate in a preferential manner.The electrons emitted by the source itself, which may be very hot, alsotake part in this process.

The vapor source described above is extremely clean, in the sense thatit does not introduce any impurity due to a crucible. If the vapor mustbe very pure, the concentrating chamber 4 maybe made of the samematerial as the cylinder 1. Also, as a result of the phenomenon of ionpumping already mentioned, the

vapor is particularly free of foreign atoms or molecules,

for example those provided by residual gases.

It should be noted that the substrate is disposed on the side of thechamber away from the electron beam, so that its electric field does notdisturb this beam. To prevent drops of liquid from the material of thesource from being sprayed on to the substrate, a grounded screen 15 maybe inserted between the source and the substrate, to prevent materialfrom following a straight path between the source and the substrate.

This screen, which may be in the form of a grating, also has the greatadvantage of placing the substrate in a Faraday cage. When the substrateis not conductive, therefore its potential can still be higher than thatof the surrounding space, sinceit is free of the negative potential dueto the electron beam.

If the temperatures to be used are very high, a heat shield can beinserted between the chamber and the vacuum enclosure in order toprotect the latter. This screen may be in the form of copper sheetcooled by fluid flowing along fluid-tight pipes bearing on it.

The dimensions of the chamber 4 are as small as possible, but they mustbe compatible with the dimensions and shape of the substrate and thedimensions of the source, so that the potential applied to the substratedoes not interfere with the beam. The dimensions of the source depend onthe energy available in the beam.

Since the chamber is already at a certain temperature due to radiationform the source and substrate, and since this temperature is fairly nearthe critical temperature for recoil, it may readily be brought to thelatter temperature by additional means. Sometimes it is only necessaryto cover or surround the chamber 4 with an additional shielding casing,by means of which the heat losses of the chamber can be reduced cheaply.

It is an advantage of the above method that it can simultaneously giveadhesion which is adjustable as desired, from almost zero adhesion ofthe layer deposited to perfect adhesion corresponding tointerpenetration of the crystal lattice of the deposit and the substratematerial, a monocrystalline deposit crystallized along the densestlattice planes, and a deposition rate of the order of l millimeter perhour.

As regards precious metals, easy recovery without special chemicaltreatment of any material which, in spite of precautions, has beendeposited on the walls of the chamber 4 while the latter has not been atthe critical temperature for recoil is facilitated if the chamber ismade from the material which is to deposited.

The arrangement according to the invention also has the advantage ofpermitting evaporation of toxic or radioactive substances withoutspecial precautions, since the vapor remains confined within the chamber4.

lclaim:

1. Apparatus for producing monocrystalline deposits on a substrate byvacuum evaporation, comprising:

a chamber within a vacuum enclosure, said chamber comprising the samematerial which is to be deposited, and said chamber containing asubstrate and a grounded screen near the substrate, said screen havingapertures therein opposite those parts of the substrate where a depositis desired,

a vertically extending shaft mounted for rotation about its axis,

a cylinder of the source material to be evaporated mounted on the upperend of said shaft, said cylinder projecting only partially into thechamber,

said chamber having an opening therein opposite the cylinder of sourcematerial for permitting an electron beam to enter the chamber and hitthe cylinder of source material, said cylinder of source material beingpositioned off center with respect l0

1. Apparatus for producing monocrystalline deposits on a substrate byvacuum evaporation, comprising: a chamber within a vacuum enclosure,said chamber comprising the same material which is to be deposited, andsaid chamber containing a substrate and a grounded screen near thesubstrate, said screen having apertures therein opposite those parts ofthe substrate where a deposit is desired, a vertically extending shaftmounted for rotation about its axis, a cylinder of the source materialto be evaporated mounted on the upper end of said shaft, said cylinderprojecting only partially into the chamber, said chamber having anopening therein opposite the cylinder of source material for permittingan electron beam to enter the chamber and hit the cylinder of sourcematerial, said cylinder of source material being positioned off centerwith respect to the opening for an electron beam, means for heating thewalls of the chamber and the substrate, means for electrically groundingthe walls of the chamber and the cylinder of source material, and meansconnecting the substrate to the positive terminal of a power sourcewhose other terminal is grounded.